A thermal battery comes in the form of a metal box which is completely sealed, is generally cylindrical, and is fitted on one of its faces with user terminals.
A thermal battery contains the same essential elements as a conventional battery: a metal cathode, a metal anode, and insulating plates impregnated with a depolarizer or electrolyte, and assembled together in cells that are identical. A plurality of such cells are stacked inside the box which is sealed by means of a lid bearing the user terminals and which is secured by crimping and then by welding.
The special feature of a thermal battery lies in that its electrolyte is in the form of a solid, and no electro-motive force (e.m.f.) can be established until the electrolyte has been fused. The electrolyte is fused, i.e. the battery is activated, by igniting disks made of a pyrotechnical composition placed between each adjacent pair of unit cells in the battery, and triggered by an electrical detonator.
This physical special feature of thermal batteries gives rise, in particular, to unusual electrical characteristics such as a very large amount of variation in the two conventional parameters of a battery: namely its e.m.f. and its internal resistance, with this variation taking place over a short period of time, of the order of a few tens to a few hundreds of seconds, between the battery being triggered and the battery coming to the end of its active life.
This very short lifetime during which the thermal battery is active constitutes a severe drawback while developing and testing devices that are designed to run on such batteries. During development and testing, it is necessary to be able to power such devices, sometimes for several hours at a time, and using genuine thermal batteries would give rise to prohibitive costs during testing. In addition, the restrictive safety conditions that accompany the use of such pyrotechnical devices and the difficulties involved in implementing a component of that type make it very difficult, if not impossible, to use a thermal battery during development and testing. Thus, during tests., a manufacturer is reduced to replacing the thermal battery with such test equipment as may be available.
Three types of equipment are presently available for replacing thermal batteries:
A power supply that is constant, using series regulation or a chopper circuit. PA1 A power supply that is programmable over a standardized IEEE 488 bus. PA1 A power supply that is programmable by an analog reference voltage.
Such a power supply provides a steady voltage (i.e. an almost perfect e.m.f., optionally associated with an on/off switch) regardless of the load applied thereto, i.e. regardless of the current delivered, in other words: EQU U.sub.bat =E.sub.ps =Const., t, I.sub.out
Controlled by a computer, a power supply of this type delivers a voltage that is variable as a function of digital control words it receives, as interpreted by the standardized coupler of the apparatus, in which case: EQU U.sub.bat =E.sub.ps =k.sub.1 i, I.sub.out, k.sub.1 i .epsilon.
The reference signal may be provided, for example, by a random signal generator, with the output voltage of the power supply being capable of describing any variable waveform (reference image) that is modifiable at will, but that is still independent of the applied load, in other words: EQU U.sub.bat =E.sub.ps =k.sub.2 C.sub.Eg (t), I.sub.out, k.sub.2 .epsilon.
Regardless of the solution chosen from the above, the voltage delivered is always a nearly perfect e.m.f., even if it does vary over time. Under no circumstances can the current delivered give rise to a change in the output voltage level, and this is not representative of the behavior of a thermal battery.
It is therefore clear that there exists a need for a system that is capable of simulating the behavior of thermal batteries more faithfully so as to facilitate developing equipment that is designed to run on thermal batteries: e.g. power supply converters or control actuators.